The field of the invention relates to the determination of intrinsic viscosity through the use of a capillary viscometer or the like.
Presently known capillary viscometers include substantially transparent tubes through which flows the liquid whose viscosity is to be measured. The tube employed in connection with the viscometer is maintained at a constant temperature. Viscosity may be measured by determining the time in which it takes a given volume of liquid contained within the tube to flow between two points, such time being proportional to the viscosity of the liquid. A pair of light sources and corresponding receivers are provided for detecting the passage of the meniscus through these points.
Because conventional capillary viscometers using fiber-optic sensors can only measure flow times between selected points to a useful absolute accuracy of .+-. 0.01 seconds, it has remained a standard practice to make viscosity measurements as a function of concentration and then extrapolate the reduced viscosity to zero concentration to determine the intrinsic viscosity. The error involved in such extrapolation processes can be decreased by, for example, increasing the number of data points or increasing the measurement time. Such steps are disadvantageous in that they are very time consuming. Attempts have also been made to increase flow time accuracy by increasing the volume of the sample.
Conventional fiber-optic probes have been used to detect the presence of the fluid meniscus within the tube of a viscometer. Such probes include two optical fiber bundles, one for transmitting light to the capillary and the other for detecting the scattered or transmitted light at the liquid-air interface. A light sensitive element produces an output signal as the meniscus falls through the field of view of the detecting fiber bundle. This signal is used to start a counter. A second such element is used to stop the counter, thereby allowing the time elapsed between the two detection points to be measured. In order for the time to be accurately determined, however, the triggering of the counter must be done at precise and repeatable points. One of the most important considerations in the design of pulse shaping and timing circuits is therefore how to produce a suitable triggering point from the output signals in order to start and stop the counter.
One approach for providing an accurate trigger point has been to employ a threshold detection circuit. However, supply voltage fluctuations produce different saturation levels in the output signals, which in turn affect the time at which such signals reach the desired threshold levels. In addition, the threshold level will have a finite ripple.